1. Field of the Invention
This invention relates to a disc drive device suitable for application to a write-once, read-many-times disc device or the like, for example. This invention relates more specifically to a disc drive device wherein upon verify for checking whether data written into a disc medium can be properly read out, for example, an amplification factor of a reproduced signal is set smaller than usual, a window width of a comparator for bringing the reproduced signal into binary form is set larger than usual, and a verify check is made based on the detected number of synchronous pattern codes from the reproduced signal as well as on error information about read data, whereby the accuracy of verify is enhanced.
2. Description of the Related Art
When a disc medium such as an optical disc, a magneto-optic disc or the like suffers large or many defects on its recording surface, there may be cases in which even if an ECC (Error Correction Code) corresponding to a redundant word is added to data and the data is written into the disc medium, error correction cannot be made thereto so that the data cannot be read out properly. Therefore, a process for checking whether the data written into the disc medium can be read properly, is executed. This process is called "verify". When the data cannot be read correctly upon verify, a process for writing the data into another area or region (alternate region) is executed.
Meanwhile, a WORM (Write Once Read Many type) disc using a pit generation system for recording marks thereon, for example, has the potential that when data is written into the disc with low recording power (mark edge recording is done), marks such as pits or the like recorded after the elapse of a few days in time deteriorate, and the data which can be properly read immediately after the writing of the data cannot thereafter be read.
FIG. 7 shows the typical relationship between recording power, an error rate and signal amplitude employed in the WORM disc. That is, a solid line a indicates the relationship between recording power and an error rate immediately after the recording of data, a broken line b indicates the relationship between recording power and an error rate after a few days since the recording, and a solid line c indicates the relationship between recording power and the amplitude of a reproduced signal immediately after the recording. Ro indicates the upper limit of an error-correctable error rate.
There may be cases in which when the recording power is placed in a small bubble shaped region Fa, the error rate is low immediately after the recording and the error correction is made possible well. Since reflectivity changes due to the occurrence of a phase change or the like although pits are not defined in the WORM disc as marks, for example, it is possible to read data. However, when the recording power falls within the bubble shaped region Fa, the phase change is restored to the original state a few days later and even recording traces of data are not left behind, whereby the data can be read. Incidentally, the amplitude of a reproduced signal at the time that the recording power lies within the bubble shaped region Fa, results in one-fourth or less the amplitude of a reproduced signal at the time that the recording power lies within a normal region Fc.
Even when the recording power is placed within a region Fb between the bubble shaped region Fa and the normal region Fc, the error rate is low immediately after the recording and the data can be read in a manner similar to the case in which the recording power lies within the bubble shaped region Fa. However, there is a potential that the error rate becomes high a few days later and the data cannot be read. This reason is considered as follows:
When data Dr shown in FIG. 8A is recorded with the recording power in the normal region Fc by using the pit generation system, for example, a pit 101 set as a record mark is defined in the WORM disc in a length corresponding to the data Dr as shown in FIG. 8B. On the other hand, when the data Dr shown in FIG. 8A is recorded with the recording power in the region Fb, a pit 102 used as a record mark is defined in the WORM disc immediately after the recording as shown in FIG. 8C. However, the periphery (shown by hatching) of the pit 102 remains in a phase-changed state. This portion is restored to the original state with the elapse of time and the pit 102 becomes short a few days later as shown in FIG. 8D, thus making it impossible to read the data. Incidentally, the amplitude of the reproduced signal at the time that the recording power lies within the region Fb, is not so reduced as compared with the case in which the recording power is placed in the normal region Fc.